The domain of the invention is that of frequency doubling devices. More precisely, the present invention concerns a low noise frequency doubler which in particular can function at frequencies of the order of several GHz, even several tens of GHz.
Frequency doubling devices which can operate at these frequencies are used in particular in radar and in instruments generating and synthesizing frequencies in order to generate stable high frequencies from low frequency source which are also stable.
The low frequency (LF) signal may for example be obtained from a quartz oscillator which provides a very stable low frequency. The use of frequency doubling devices in series leads to a rapid increase in the frequency of the signal and provides a high frequency signal of stable frequency. In order to reduce the number of multiplications necessary to reach the desired high frequency, it is advantageous to have available a stable LF source with relatively high frequency.
Most frequency doubling devices used are based on the introduction of a distortion of an input sine wave signal at frequency F to enrich the spectrum and thus create harmonic frequencies of the input frequency: 2F, 3F, 4F, etc. Selective filtering then enables the desired harmonic to be recovered, provided it was generated with sufficient amplitude. In the case of frequency doublers, the 2F harmonic is selected.
Depending on the operation performed to distort the input sine wave signal (chopping the top, rectification, chopping the bottom, etc.) and the way in which this is done, the desired harmonics may be generated. In general, the efficiency of this operation is more or less satisfactory and some attenuation in the phase and/or amplitude of the noise spectrum is achieved.
Different types of devices capable of generating multiple frequencies from an LF signal are known.
One may for example use a "snap-off" diode which stores all the energy of one period of a sine wave signal of frequency F applied to it, restoring the energy in the form of a Dirac pulse, i.e. in a very short time period. Since the spectral width of the signal obtained is very broad, the signal must be filtered in order to extract the harmonic of the desired frequency, for example 2F.
The main disadvantage of this type of device is that the noise level introduced is relatively high and is not suitable for low noise applications.
Another method used to increase the frequency of a signal makes use of the fact that an input signal of frequency F multiplied by itself provides a signal whose amplitude is equal to the square of that of the input signal and whose frequency is doubled. Since the multiplication of a signal by itself enables the frequency to be doubled, a signal of several tens of GHz can be obtained from a low frequency signal of several tens of MHz.
One can for example use a mixer receiving two identical signals of the same frequency, but the multiplication operation is not linear and the 2nd order harmonic is not the only one created; the spectrum is thus uselessly enriched.
Another type of frequency doubler is shown in FIG. 1. This figure shows a frequency doubler using full-wave rectification.
An input sine wave signal V.sub.e of frequency F is applied to the input of the device at the primary winding of a transformer 12 whose outputs are symmetric differentials relative to the mid-point of the secondary of transformer 12, connected to the anode of a diode D whose cathode is grounded. A capacitor C is connected to diode D in parallel and the two components cooperate to maintain a voltage equal to the threshold voltage of the diode on the mid-point of the secondary of transformer 12. Diode D is energized by a current with voltage +V through a resistor R. Each of the symmetric differential outputs of transformer 12 is connected to the base of bipolar transistors T.sub.1,T.sub.2. The emitters of transistors T.sub.1 and T.sub.2 are connected to ground and their collectors are connected to a load resistor Rc. The common point of the two transistors T.sub.1 and T.sub.2 (point A) is connected to the input of filter 10. The output signal Vb of the filter 10 is a sine signal whose frequency is 2F.
The frequency doubler operates as follows: the signals applied to the bases of transistors T.sub.1 and T.sub.2 are in phase opposition, so each transistor alternatively amplifies the input signal while the other transistor is non-conducting. Diode D and capacitor C change the threshold of the transistors to compensate their V.sub.be. The threshold voltage of the diode is generally about 0.6 to 0.7 volts. The operation of the frequency doubler is thus a class B push-pull operation (symmetrical). Signal Va at point A is formed from juxtaposed rectified sinusoidal half-cycles, since transistors T.sub.1 and T.sub.2 alternatively generate one half-cycle of the sine wave. The junction between adjacent half-cycles of the sine wave is abrupt, signal Va having switching points 11 carrying very high frequency spectra. Filter 10 eliminates the base frequency F and provides the output signal Vb of frequency 2F. The gain of this arrangement is constant since the emitters are connected to ground and depend on the types of transistors T.sub.1 and T.sub.2 (dynamic resistance value of the emitter).
The use of several modules of this type in series enables the frequency of an input signal to be considerably increased, the first module supplying 2F at its output, the second 4F, the third 8F, and so on.
Nevertheless, since full wave rectification creates switching points 11 with very rich spectra (lines of several GHz are observed for an input signal V.sub.i of the order of 150 MHz), it is not possible to control the spectrum totally and leaks are observed, which are responsible for parasitic phenomena.
These parasitic phenomena make the device sensitive to vibrations when it is mounted in a unit which is for example part of a chain (connection in series) of frequency doublers. In addition, the parasitic frequencies may create standing waves in the unit and mechanical vibrations may modulate these waves. The shape of the output signal V.sub.b is thus considerably affected.
Furthermore, a filter to select a harmonic occupies non-negligible space in units composing a chain of frequency doublers and problems of size are common in instruments using this type of frequency doubler.
Nor is it possible to modify, at least more than slightly, the frequency of the input signal, since the filter is centered on a fixed frequency. Thus, if we wish to obtain a different output frequency from a chain of frequency doublers or change the frequency F of the basic input frequency of the chain, it is necessary to change all the filters in the frequency doublers of the chain.
An additional disadvantage arising from the presence of a filter is the increased cost of such a frequency doubler.